dave@onfcanim.UUCP (Dave Martindale) (01/08/89)
Laser speckle is a particularly special case of interference, because it happens in your eye, not on the surface that the laser is hitting. A ray-tracing system that dealt with interference of light from different sources would show the interference fringes that occur when a laser light source is split into two beams and recombined, and the interference of acoustic waves. But to simulate laser speckle, you'd have to trace the light path all the way back into the viewer's eye and calculate interference effects on the retina itself. If you don't believe me, try this: create a normal two-beam interference fringe pattern. As you move your eye closer, the fringes remain the same physical distance apart, becoming wider apart in angular position as viewed by your eye. The bars will remain in the same place as you move your head from side to side. Now illuminate a target with a single clean beam of laser light. You will see a fine speckle pattern. As you move your eye closer, the speckle pattern does not seem to get any bigger - the spots remain the same angular size as seen by your eye. As you move your head from side to side, the speckle pattern moves. As the laser light reflects from a matte surface, path length differences scramble the phase of light travelling by slightly different paths. When a certain amount of this light is focused on a single photoreceptor in your eye (or a camera), the light combines constructively or destructively, giving the speckle pattern. But the size of the "grains" in the pattern is basically the same as the spacing of the photorecptors in your eye - basically each cone in your eye is receiving a random signal independent of each other cone. The effect depends on the scattering surface being rougher than 1/4 wavelength of light, and the scale of the roughness being smaller than the resolution limit of the eye as seen from the viewing position. This is true for almost anything except a highly-polished surface, so most objects will produce speckle. Since the pattern is due to random variation in the diffusing surface, there is little point in calculating randomness there, tracing rays back to the eye, and seeing how they interfere - just add randomness directly to the final image (although this won't correctly model how the speckle "moves" as you move your head). However, to model speckle accurately, the pixel spacing in the image has to be no larger than the resolution limit of the eye, about half an arc minute. For a CRT or photograph viewed from 15 inches away, that's 450 pixels/inch, far higher than most graphics displays are capable of. So, unless you have that sort of system resolution, you can't show speckle at the correct size.